In recent years, traditional cathode ray tube (CRT) displays have been gradually replaced by liquid crystal displays (LCD), which are light-weighted, thin, short and small in size and almost radiation free, and have low heat and power consumption.
In general, the primary structure of a LCD includes a panel board and a backlight module. The panel board includes, for example, indium tin oxide (ITO) conductive glass, liquid crystals, an alignment film, a color filter, a polarizer and a drive integrated circuit. The backlight module includes lighting tubes, a light guide and various optical films. The entire optical performance of a LCD is interactively influenced by all the elements of the backlight module, among which the optical films play an important role in brightness, evenness, contrast and viewing angle of the screen.
Various attempts have been made in the industry to improve the viewing angle and color of LCDs. For example, it is known that the brightness of a LCD panel board may be enhanced by increasing the numbers of lighting tubes, which, however, not only tends to cumulate excessive heat in the LCD and will influence the lifetime and quality of other elements, but also consumes a large amount of electricity, and thus fails to meet the requirement that many IT products have to depend on batteries for off-line applications.
Currently, the most economic and convenient means has been using various optical films in the backlight module to enhance the brightness of the panel board of a LCD so as to maximize the efficiency of the light source without any change in element designs or consumption of additional energy.
“Brightness enhancement film” is commonly abbreviated as “BEF” or referred to as “light-focusing film,” which may be made, for example, by curing a special acrylic resin into a micro-prism structure with high energy ultraviolet radiation (UV) on a polyester optical film having a thickness of 125 μm only. The major function of the brightness enhancement film is to collect the dispersive lights emitted towards all the directions from the light guide by refraction and total internal reflection, and to focus the lights in the on-axis direction of about ±35 degrees, so as to enhance the brightness of the LCD.
There have been published patent documents disclosing the employment of light-focusing films in backlight modules. For example, U.S. Pat. No. 4,791,540 granted to 3M Corp. discloses using a backlight module comprising more than two perpendicular overlapping light-focusing films to effectively enhance the brightness. In addition, U.S. Pat. No. 5,828,488 discloses using a backlight module comprising a diffusion film and a light-focusing film to effectively enhance the brightness of the LCD panel board.
Common light-focusing films are known to have prism peaks. If the prism peaks do not have sufficient hardness, collapse will take place, which will result in dark spots on the appearance of the LCD panel board. Moreover, such prism peaks are normally vulnerable, and thus prone to be scratched during assembly or transportation. Therefore, how to increase the hardness of the prism peaks of a light-focusing film has become an important research project in the industry.
This invention provides a solution for the project.